The present disclosure relates to an optical fiber sensor and a seismic prospecting method using the same, a method of measuring distribution of a petroleum/natural gas reservoir layer, a strain detection method, and a method of specifying a position of a fissure in a stratum.
Seismic prospecting is frequently used when searching for petroleum gas. Sensing with an optical fiber has been performed for prospecting as a method of measuring an acoustic wave propagating in a deep underground and a seabed in recent years. A common acoustic wave optical fiber sensing uses an interference type sensor using a Michelson interferometer (Proceedings of 32nd Meeting on Lightwave Sensing Technology LST32-13, pp. 93-98) and an optical time domain reflectometry method (OTDR) specifying a position of a failure point by introducing an optical pulse into an optical transmission path and observing back-scattered light generated from the optical pulse.
The OTDR is widely used for managing and monitoring an optical fiber communication transmission path as a method of exploring a fracture point and a failure point formed in the optical transmission path (Journal of Lightwave Technology, VOL. LT-3, No. 4, pp. 876-886). In this measurement method, a distance resolution (the minimum distance of specifying a discrete phenomenon in the optical fiber) of a reflection point in the longitudinal direction of the optical fiber is defined by a distance by which an incident optical pulse travels for a duration of its pulse. Therefore, the distance resolution is ordinarily about 100 to 10 m and at least about 1 m. In the common acoustic wave optical fiber sensing by the OTDR, the distance resolution is at least about 1 m.
In addition, as a measurement method achieving the higher distance resolution than the OTDR, an optical frequency-domain reflectometry (OFDR) is used for a fiber optical communication technology. The OFDR sweeps a frequency of light emitted by an light source linearly in time, thereafter demultiplexes the light to signal light and reference light, introduces the signal light into an optical fiber subjected to measurement, multiplexes back-scattered light generated in the subject optical fiber with the reference light, and performs a heterodyne detection to the multiplexed light. A frequency difference between the back-scattered light and the reference light is in proportion of a difference in their optical path lengths. Therefore, a position where the back-scattered light is generated in the optical fiber may be specified by measuring a reflected-light intensity distribution in a frequency domain. In the OFDR, the distance resolution may be improved by narrowing a measured frequency band width at a receiving unit (Japanese Laid-open Patent Publication No. H5-203410).
Although the acoustic wave optical fiber sensor used as a conventional technology for petroleum gas prospecting uses the OTDR, a measurement method and a device with higher distance resolution are demanded to improve accuracy for petroleum gas.